<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .slider-bar {
            position: absolute;
            left: 50%;
            top: 300px;
            margin-left: 600px;
            width: 45px;
            height: 130px;
            background-color: pink;
        }
        
        .w {
            width: 1200px;
            margin: 10px auto;
        }
        
        .header {
            height: 150px;
            background-color: purple;
        }
        
        .banner {
            height: 250px;
            background-color: skyblue;
        }
        
        .main {
            height: 1000px;
            background-color: yellowgreen;
        }
        
        span {
            display: none;
            position: absolute;
            bottom: 0;
        }
    </style>
</head>

<body>
    <div class="slider-bar">
        <span class="goBack">返回顶部</span>
    </div>
    <div class="header w">头部区域</div>
    <div class="banner w">banner区域</div>
    <div class="main w">主体部分</div>
    <script>
        var header = document.querySelector(".header");
        var span = document.querySelector(".goBack");
        var slider = document.querySelector(".slider-bar");
        var banner = document.querySelector(".banner");
        var slidertop = slider.offsetTop - banner.offsetTop;
        document.addEventListener("scroll", function(param) {
            if (window.pageYOffset >= header.offsetHeight) {
                span.style.display = "block";
                slider.style.position = "fixed";
                slider.style.top = slidertop + "px";
            } else {
                span.style.display = "none";
                slider.style.position = "absolute";
                slider.style.top = "300px";
            }
        })
    </script>
</body>

</html>